Flow meter assembly, gate assemblies and methods of flow measurement

ABSTRACT

The present invention provides an acoustic flow meter assembly ( 2 ) for pipes or open channels, said assembly including a frame ( 24 ) with a predetermined geometry. The frame has at least one user accessible port ( 36 - 42 ) with the at least one user accessible port ( 36 - 42 ) adapted to receive an interchangeable cartridge ( 44 ) which contains at least one acoustic transducer ( 46 ) to measure fluid velocity through said frame ( 24 ).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/399,957, filed Feb. 17, 2012 and entitled “Flow Meter Assembly, GateAssemblies and Methods of Flow Measurement;” which is a continuation ofInternational Application No. PCT/AU2010/001052 filed Aug. 18, 2010 andpublished as WO 2011/020143 A1, entitled “Flow Meter Assembly, GateAssemblies and Methods of Flow Measurement;” which claims priority toAustralian Patent Application Serial No. 2009903893, filed Aug. 18,2009; Australian Patent Application Serial No. 2009905149, filed Oct.22, 2009; and Australian Patent Application Serial No. 2010902414, filedJun. 2, 2010. All of the foregoing applications are hereby incorporatedby reference herein.

FIELD OF THE INVENTION

This invention relates to an acoustic flow meter assembly for pipes oropen channels and relates particularly, though not exclusively, to anacoustic flow meter assembly for monitoring water flow. The inventionalso relates to an undershot gate leaf assembly which may be used withthe acoustic flow meter assembly.

DESCRIPTION OF THE PRIOR ART

Flow meters are commonly used to measure the flow rate of fluids withinburied pipes and open channels or culverts. Transit time acoustic flowmeters are an established measurement technology. When flow meters areinstalled in pipes which are below ground, servicing requirements meanthat these flow meters are traditionally installed within a buried meterpit, typically a concrete box construction. The pit is typicallyaccessible so that technicians may access the components of the flowmeter. The construction and installation of these service pits isgenerally a high proportion of the total flow meter installation cost.

When ultrasonic (transit time) flow meters are installed in openchannels and pipes they are typically installed as a collection ofsub-components which must be assembled and then calibrated to theirinstallation. The commissioning of these metering systems requires theprecise measurement of the path length between each transducer, theangle of the measurement path relative to the mean direction of flow,and of water level transducer datum's and other meter configurationparameters. Other acoustic flow meter products available in themarketplace are assembled on site by strapping the acoustic transducersaround the external or internal diameter of the pipe which passes theflow. In open conduit applications the transducers are bolted to theopposing walls of the conduit. The transducers are connected by signalcables to processor electronics. The assembly must be precisioninstalled and calibrated in the field. For installations in which thetransducers are installed on the internal diameter of the pipe, the pipemust be of sufficient diameter that a person may safely access it forthe purpose of installation. For installations in which the transducersare installed on the outer diameter of the pipe, the pipe must be aboveground or a large concrete pit must be constructed around the pipe topermit a person to safely access the external diameter for the purposeof fitting and maintaining the sensors.

In open channel flow meter applications, the accuracy of the flow meteris affected by the flow meter surroundings. The geometry of the channelupstream and downstream of the flow meter can influence the velocitydistribution of the fluid passing through the flow meter. This velocitydistribution is measurable at all points within the flow meter exceptfor the surface. The velocity of the fluid on the floor/walls of theflow meter is zero. The velocity at set elevations within the flow metercan be measured, and the velocity at elevations between thesemeasurements can be interpolated from the measured elevation velocities.However generally the surface velocity of the flow is not measured andso the velocity distribution in the upper levels of the flow must beextrapolated with potentially high uncertainty. To minimise theuncertainty in the surface velocity of the flow, the variation insurface velocity behaviour needs to be minimised.

OBJECTS OF THE INVENTION

It is an object of the present invention to reduce the infrastructurecosts of a flow meter installation to allow installation of more flowmeters which provide more data to be gathered to locate distributionsystem losses.

A further object of the invention is to provide a flow meter whichcompletely defines its own geometry and does not require calibration toits installation or surroundings.

In another object of the invention there is provided an undershot flowgate which influences the flow profile to create non-turbulent,streamlined and repeatable flow behaviour.

Yet another object of the invention is to provide a flow meter, for usein a closed conduit, which includes a gate valve or the equivalent, butwithout what is referred to as a “bonnet” of the type which constitutean integral component of a traditional gate valve.

SUMMARY OF THE INVENTION

With these objects in view the present invention provides an acousticflow meter assembly for pipes or open channels, said assembly includinga frame with a predetermined geometry, said frame including at least oneuser accessible port, said at least one user accessible port adapted toreceive an interchangeable cartridge which contains at least oneacoustic transducer to measure fluid velocity through said frame.

Preferably the acoustic flow meter assembly further includes a pluralityof user accessible ports with an associated cartridge. The useraccessible ports may be located in corners of a rectangular or squareorientation formed by said frame. Preferably a pair of cartridges arediagonally directed towards each other.

In a preferred embodiment each cartridge includes a plurality ofacoustic transducers for measuring flow at predetermined depths. Theacoustic flow meter assembly may further include a hollow tube forcoupling at either end to a pipeline to determine the velocity throughsaid pipeline. In a practical embodiment each transducer is located atone end of a respective sound transmission tube and the other end opensinto said hollow tube. Each sound transmission tube can be associatedwith a respective cartridge and angled towards an associated facingsound transmission tube. Each sound transmission tube may contain fluidfrom said hollow tube. Each sound transmission tube may contain stillfluid which is not in the path of the fluid flow.

In a further embodiment each sound transmission tube is filled with anacoustic transmissive material. The acoustic flow meter assembly mayfurther include a boundary interface between the fluid in said soundtransmission tube and the flowing fluid, said boundary interface formedof a material of suitable acoustic properties to enable readytransmission of the acoustic signals. The fluid in the soundtransmission tubes may also be contained in a sealed well such that thefluid couples the transducers to the inner face of the soundtransmission tubes.

The invention may also provide a tilt lift gate assembly including agate member which can be raised and lowered from a vertically closedposition through to a substantially horizontal disposition, said gatemember being pivotally mounted at the top end thereof to a mechanism forpulling said gate member inwardly from the vertically closed position tothe substantially horizontal disposition and at least one extensionprojecting from said gate member with a pivot point at the end of saidat least one extension, said pivot point co-operating with a downwardlyangled guide means whereby movement of said gate member does not crosssaid downwardly angled guide means.

It is preferred that a pair of extensions are located on each side ofsaid gate member which co-operate with respective downwardly angledguide means. The tilt lift gate assembly may be located in an open fluidchannel, said at least one extension being positioned substantially twothirds of the depth of the fluid.

The invention may also provide an open channel fluid velocity system formeasuring the fluid velocity of the fluid flowing through said system,said system including an open channel containing said flowing fluid, anacoustic flow meter assembly as previously described and a tilt liftgate assembly as previously described downstream of said acoustic flowmeter assembly, wherein said gate member predictably influences thesurface velocity of said flowing fluid.

The invention may also provide an open channel fluid velocity system formeasuring the fluid velocity of the fluid flowing through said system,said system including an open channel containing said flowing fluid, anacoustic flow meter assembly as previously described and an undershotgate downstream of said acoustic flow meter assembly, wherein said gateallows the fluid level in front of said gate to back to provide auniform depth of fluid through said acoustic flow meter assembly.

The invention may also provide a method of measuring fluid velocity in apipe or open channel, said method including the steps of: providing atiming circuit which includes a first circuit having at least oneupstream acoustic transducer and a second circuit having at least onedownstream acoustic transducer, measuring the time delay in detectingthe acoustic signal from said at least one upstream acoustic transducerto said at least one downstream acoustic transducer from said firstcircuit, measuring the time delay in detecting the acoustic signal fromsaid at least one downstream acoustic transducer to said at least oneupstream acoustic transducer from said second circuit, measuring thetime delay in said first circuit when said at least one upstreamacoustic transducer is bypassed in said first circuit, measuring thetime delay in said second circuit when said at least one downstreamacoustic transducer is bypassed in said second circuit, and calculatingthe fluid velocity using said measurements.

In yet a further aspect of the invention there may be provided anacoustic flow meter for a pipe, said assembly including at least threepairs of acoustic transducers, each pair of said acoustic transducerslocated on opposing sides of said pipe and offset longitudinally alongsaid pipe to provide upstream and downstream transducers, each pair ofacoustic transducers, in use, having their acoustic paths intersectingat a point along the axis of said pipe to provide redundancy inmeasuring flow through said pipe if one of said acoustic transducersshould fail.

The invention also provides a lift gate assembly including a gate memberassociated with a frame and which can be raised and/or lowered frombetween respective closed and open configurations, said frame havingassociated therewith and upstream thereof an apparatus for measuringtransit turn of fluid, said apparatus being in the form of a conduithaving one or more opposed pairs of acoustic transducers or the likeassociated therewith.

In another aspect there is provided a method of measuring acoustictransit times in an open channel or river, said method including thesteps of: providing a first circuit having at least one upstreamacoustic transducer on one side of said open channel or river and asecond circuit having at least one downstream acoustic transducer on theopposite side of said open channel or river, said first and secondcircuits including respective timing circuitry which are notsynchronised with one another, each of said timing circuits measuringtheir respective signal transmit and receive events, at least one ofsaid first or second circuits including an RF or laser to providesynchronising signals between said first and second circuits, an RF orlaser synchronising signal is transmitted between said first and secondcircuits prior to an acoustic signal transmitted from one of saidacoustic transducers between said first and second circuits whereby saidRF or laser synchronising signal allows synchronisation between therespective timing circuitry of said first and second circuits of saidacoustic signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood and put intopractical effect, reference will now be made to the accompanyingdrawings, in which:—

FIG. 1 is an exploded perspective view of a first embodiment of anacoustic flow meter assembly for a pipe made in accordance with theinvention;

FIG. 2 is a diagonal cross-sectional view of the acoustic flow meterassembly shown in FIG. 1 in its buried position in the ground;

FIG. 3 is a front view of the acoustic flow meter assembly shown in FIG.1;

FIG. 4 is a plan view of the acoustic flow meter assembly shown in FIG.1;

FIG. 5 a is a longitudinal cross-sectional view along and in thedirection of arrows 5-5 of the acoustic flow meter assembly shown inFIG. 1 showing water flow;

FIG. 5 b is a similar view to that of FIG. 5 a with filled soundtransmission tubes;

FIG. 6 a is longitudinal view of water flow showing the velocityprofile;

FIG. 6 b is a cross-sectional view along and in the direction of arrowsA-A of FIG. 6 a;

FIG. 7 is a perspective view of a second embodiment of an acoustic flowmeter assembly for use in an open channel environment;

FIG. 8 is a plan view of FIG. 7;

FIG. 9 is a cross-sectional along in the direction of arrows A-A shownin FIG. 8;

FIG. 10 is a similar view to that of FIG. 6A but showing the embodimentof the acoustic flow meter assembly shown in FIG. 7 being used incombination with a vertically raiseable undershot gate leaf forcontrolling water flow;

FIG. 11 is a similar view to that of FIG. 10 but having a rotatableundershot gate leaf;

FIG. 12 is a perspective view of an embodiment of the construction ofthe rotatable undershot gate leaf assembly depicted in FIG. 8 in theclosed position;

FIG. 13 is a similar view to that of FIG. 12 with the gate leaf beingraised;

FIG. 14 is a similar view to that of FIG. 13 with the gate leaf in thefully raised position;

FIG. 15 is a longitudinal cross-sectional view of FIG. 12;

FIG. 16 is a longitudinal cross-sectional view of FIG. 13;

FIG. 17 is a longitudinal cross-sectional view of FIG. 14;

FIG. 18 is a similar view to that of FIG. 15 showing the water flow; and

FIG. 19 is a similar view to that of FIG. 16 showing the water flow.

FIG. 20 is a flow schematic diagram to typically control the acoustictransducers used in the embodiment of the acoustic flow meter assemblyshown in FIGS. 1 to 19 to measure the acoustic travel time betweentransducers;

FIG. 21 is a partial view of the flow schematic diagram shown in FIG. 20measuring flow in a first direction;

FIG. 22 is a partial view of the flow schematic diagram shown in FIG. 20measuring flow in a second direction opposite to that shown in FIG. 21;

FIG. 23 is a flow schematic diagram of a calibration circuit used inconjunction with the flow schematic diagram shown in FIG. 20 toeliminate circuit delays from the diagram of FIG. 20;

FIG. 24 is a partial view of the flow schematic diagram shown in FIG. 23to calibrate delays for measuring flow in the first direction shown inFIG. 21;

FIG. 25 is a partial view of the flow schematic diagram shown in FIG. 23to calibrate delays for measuring flow in the second direction oppositeto that shown in FIG. 21;

FIG. 26 is a side view of a pipe showing a further embodiment of theinvention for the measurement of fluid velocity in a pipe;

FIG. 27 is an end view of FIG. 26;

FIG. 28 is a top perspective of a flow gate including a transit timemeasuring apparatus in accordance with the invention;

FIG. 29 is a view, similar to FIG. 28, of an alternative arrangement offlow gate and transit time measuring apparatus;

FIG. 30 is a front view of the arrangement of FIG. 28;

FIG. 31 is a sectional view taken along the line A-A in FIG. 30, withthe gate closed;

FIG. 32 is a sectional view taken along the line A-A in FIG. 30, withthe gate open;

FIG. 33 is a sectional view taken along the line B-B in FIG. 30;

FIG. 34 is a detail view taken at E in FIG. 33;

FIG. 35 is a detail view taken at D in FIG. 32;

FIG. 36 is a detail view taken at B in FIG. 31;

FIG. 37 is a reduced view similar to that of FIG. 28 with a divider inthe measuring apparatus;

FIG. 38 is a similar view to that of FIG. 37 with 2 dividers in themeasuring apparatus;

FIG. 39 is a reduced view similar to that of FIG. 29 with a divider inthe measuring apparatus;

FIG. 40 is a similar view to that of FIG. 39 with 2 dividers in themeasuring apparatus;

FIG. 41 is a top perspective of a flow gate similar to FIG. 28 having aslanted control gate;

FIG. 42 is a reduced view similar to that of FIG. 41 with a divider inthe measuring apparatus;

FIG. 43 is a similar view to that of FIG. 42 with 2 dividers in themeasuring apparatus;

FIG. 44 is a plan view of a further embodiment to measure the acoustictravel time between transducers using a radio transmitter;

FIG. 45 is a plan view of an embodiment similar to that shown in FIG. 44to measure the acoustic travel time between transducers using lasers;

FIG. 46 is a vertical cross section of the embodiment shown in FIG. 44;and

FIG. 47 is a perspective view of a sealed cartridge containing theelectronics for the embodiments shown in FIGS. 44 to 46.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout this specification the same reference numerals will beutilised, where applicable, to avoid repetition and duplication ofdescription across all embodiments. The description of constructions andoperation will be equally applicable.

In FIGS. 1 to 6 of the drawings there is shown an acoustic flow meterassembly 20 which is adapted to be fitted between a pipeline (not shown)through which fluid flows, preferably a liquid. In this embodiment thefluid is water but the invention is not limited to such an environment.The preferred embodiments are particularly useful for the metering ofirrigation water consumption in irrigation channels in internationalirrigated agriculture regions and the metering of urban water suppliesin international urban water networks. The acoustic flow meter 20 isburied in the ground 22 (FIG. 2) and includes a frame 24 which supportsa pipe section 26. Pipe section 26 is adapted to be coupled to eitherend of the pipeline through which the flow rate is to be determined.Frame 24 in this embodiment is basically of a square shape and has twoend members 28, 30 and two side members 32, 34. The shape andconstruction of the frame 24 can vary to suit the requirements of theparticular flow meter assembly. Four hollow legs 36, 38, 40 and 42 formpart of the frame 24 and slidably receive cartridges 44 which can beinserted therein. The number and positioning of the cartridges 44 canvary depending on the environment in which flow rate is to bedetermined. In this embodiment each cartridge includes four acoustictransducers 46. The number and positioning of the acoustic transducers46 can also be varied. The acoustic transducers 46 are integrated intoelectronic circuitry (not shown) which can be included in the cartridges44 and frame 24. The serviceable components including the acoustictransducers 46 and processing electronics are all contained withinsealed cartridges 44 which can be interchanged. Typically, thecartridges 44 can provide their measurements by wired or wireless meansto an external computing device.

Pipe section 26 has number of sound transmission tubes 48 which aremounted in a horizontal disposition as clearly shown in FIGS. 2 and 3.The sound transmission tubes 48 are typically cylindrical in shape andare made of an acoustically transmissive material which couples thealigned acoustic transducers 46 to the internal bore of the pipe section26. The sound transmission tubes 48 are arranged to intersect the pipesection 26 at an angle θ (FIG. 5 a) to the direction of fluid flow 50.The preferred intersection angle θ is 45 degrees, however otherimplementations could be manufactured with an intersection angle θbetween 0 and 90 degrees to suit geometry requirements of variousapplications. The sound transmission tubes 48 provide an acoustic pathfor the acoustic transducers 46 located within the flow meter cartridges44. In FIG. 5 a the sound transmission tubes 48 are hollow so that theycontain the fluid within the pipe section 26 and the sound propagatesthrough this fluid only. The sound transmission tubes 48 will containstill water and will not be in the path of the water flow.

Alternatively, as shown in FIG. 5 b, the sound transmission tubes 48 maybe filled or plugged with a solid material of appropriate acousticbehaviour so that the pipe section 26 is completely sealed and thecartridges 44 may be retrieved while the pipe is operating under apositive or negative pressure without the requirement to seal accessports 52 against this pressure. The sound transmission tubes 48 couldalso be filled with water with a boundary interface (not shown) betweenthe still water in the sound transmission tubes 48 and the flowingwater. This interface would be made of a material of appropriateacoustic properties that enables the ready transmission of the acousticsignals. An advantage of this embodiment with the closed soundtransmission tubes 48 is that the internal bore of the pipe section 26will be smooth and there will be no potential for clogging or trappingof debris in the pipe section 26 or the sound transmission tubes 48. Inthis arrangement a good acoustic coupling would be achieved between theacoustic transducers 46 contained within the cartridges 44 and the endfaces of the sound transmission tubes 48 by employing a cammingmechanism within the access ports 52 which would positively engage theacoustic transducers 46 against the faces of the sound transmissiontubes 48.

Alternatively, a simpler coupling mechanism can be achieved by fillingaccess ports 52 with water or similar fluid which acoustically couplesthe transducers 46 contained within cartridges 44 to the end faces ofthe sound transmission tubes 48. In this implementation, the accessports 52 are a sealed well containing a fluid which couples thetransducers 46 to the inner face of the sound transmission tubes 48. Theaccess ports 52 are typically aligned vertically and accessed throughsealed lids 54 at ground level. In some applications the access ports 52might be aligned horizontally and accessed through wall mounted lids.The access ports may be installed at any other angle as the installationrequires.

Within a horizontal plane of the acoustic flow meter assembly 20 thereare four acoustic transducers 46, which are arranged to provide twoacoustic paths 58, 60 within each horizontal plane (FIGS. 5 a and 5 b).As there are four acoustic transducers in each cartridge 44 there willbe four horizontal planes 62, 64, 66 and 68 (FIGS. 6 a and 6 b). Theseacoustic paths are at right angles to each other, and this arrangementeliminates cross flow errors as discussed in Section 13.1.3 of ASTMD5389-93 (2007) Standard Test Method for Open-Channel Flow Measurementby Acoustic Velocity Meter Systems.

The acoustic transducers 46 transmit a high frequency (in the kilohertzto megahertz range) sound pulse across the pipe section 26. The traveltime of the acoustic signal is measured in a direction upstream to thedirection of flow 50, and also in a direction downstream to thedirection of flow 50 as seen in FIGS. 5 a and 5 b. The flow velocitycreates a difference in the sound wave travel times in the upstream anddownstream direction. This travel time difference is recorded and usedto determine the average velocity of the fluid along the line of theacoustic path. The four measurement paths provide an average velocity ofthe fluid at four different planes 62-68 as shown in FIG. 6 a. Thevelocity distribution within the pipe section 26 is then calculated fromthe velocities at each of the four planes 62-68 using a calibratedmathematical relationship.

A water level sensor, preferably an acoustic water level sensor 45, willbe associated with each cartridge 44. In the preferred embodiments ofFIGS. 1 to 10, for example, each cartridge 44 includes a port, generallydesignated 47, for receiving and releasably retaining an acoustic waterlevel sensor 45. It should be understood, however, that it is notessential for the water level sensor to be physically integrated into orwith the associated cartridge 45, so long as a water level sensor islocated at or in the vicinity of each cartridge 44.

The water level sensors 45 function to provide an accurate measurementof the profile of the water surface at or in the vicinity of the overallflow meter assembly. Since a measurement is being made of the averagevelocity of flow of the water, then in order to be able to accuratelycompute the volumetric flow rate an accurate measurement of thecross-sectional area of flow at the location of the flow meter assemblyis also required.

The preferred arrangements as illustrated and described, with anacoustic water level sensor 45 associated with each of the fourcartridges 44, ensures an accurate determination of volumetric flow ofwater, even in situation/circumstance wherein the surface of the wateris disturbed or uneven, as for example with there being turbulent flowor, in the alternative, a sloping surface gradient.

Other embodiments may include any number and combination of acoustictransducers 46, as required, to realise other signal pathconfigurations. The use of signal reflectors to replace some of thetransducers in each measurement plane could also be used. It is notnecessary to have four planes 62-68 across the acoustic flow meterassembly 20. Any number of planes may be used, for example, one or aplurality of planes. The planes need not be horizontal as shown in thisembodiment.

FIGS. 7 to 9 show the use of the acoustic flow meter assembly 20 in anopen channel environment, typically used for water irrigation. AU-shaped channel 70 having a base 72 and sidewalls 74, 76 is used tocontrol flow of irrigation water. The acoustic flow meter assembly 20shown in FIG. 1 can be used but does not require the access ports 52 asthe installation is not buried in the ground. Pipe section 26 is notrequired. This embodiment is similar in construction to the previousembodiment in that four retrievable cartridges 44 are provided. Howeverthe system can also be designed with one, two, three, or moreretrievable cartridges 44, similar to the previous embodiment. Theacoustic flow meter assembly 20 is manufactured under high tolerance andcompletely defines the geometry that the metered fluid passes through.This assembly 20 ensures that the fluid always passes through the samegeometry through the body of the acoustic flow meter assembly 20regardless of the geometry of the channel 70 into which it is installed.The cartridges 44 can be slidably removed and replaced without changingthe geometry of the acoustic flow meter assembly 20. The cartridges 44are each individually calibrated with a calibration referenced to theirmounting points within the four hollow legs 36, 38, 40 and 42. Thisallows the cartridges 44 to be interchanged without effecting thecalibration of the acoustic flow meter assembly 20. The acoustictransducer behaviour and geometry requirements are the same as describedfor the previous embodiment.

In FIG. 10 the acoustic flow meter assembly 20 of FIGS. 7 to 9 includesa downstream control gate 80. In this embodiment the control gate 80 isa simple guillotine gate which is raised and lowered vertically andcloses on a seal 82. The control gate 80 can be separate from theacoustic flow meter assembly 20, as shown, or could be integrated into acombined assembly. The control gate 80 forms an undershot gate whichinfluences the surface velocity of the fluid 84 flowing through theacoustic flow meter assembly 20 and reduces the influence of thesurrounding world on the flow profile passing through the acoustic flowmeter assembly 20. As previously described the velocity is measured at anumber of vertical elevations by acoustic transducers 46, and thevelocity at each of these elevations is then fitted to a relationshipwhich is used to interpolate the velocity at heights between the sampledelevations.

The surface velocity of the fluid 84 is typically not measured becausethe elevation of the surface thereof varies during operation and so itis generally not possible to locate an acoustic transducer plane at thesurface 86 of the fluid. The floor velocity is always zero, and thevelocity at all elevations below the top transducer plane 62 can beinterpolated from the measured values obtained in the planes above andbelow the elevation of interest. The unknown surface velocity means thatthe velocity at elevations above the top transducer plane 62 must beextrapolated based on assumptions of the shape of the velocity profile.This top section of the flow is typically where the greatestuncertainties in the velocity profile occur, as there is no informationabout the velocity at the surface. In worst case scenarios this velocitycould be extremely high or even in a reverse direction to the flow dueto surface influences such as wind. By locating control gate 80downstream of acoustic flow meter assembly 20 and ensuring that thelower tip 88 of control gate 80 is always submerged, the control gate 80maintains a laminar and streamlined flow profile which is free ofturbulence. The velocity of the fluid will be zero in front of controlgate 80. This flow profile is repeatable and may be characterised by aflow model which computes the flow rate using measurements of gateposition and the fluid velocities measured by the acoustic transducersystem. The repeatability of the flow profile passing under the controlgate 80 is combined with the measured flow velocities at each of thesensor plane elevations 62, 64, 66 and 68 and is used to reduce theuncertainty in the estimation of the fluid's surface velocity throughthe body of the acoustic flow meter assembly 20.

The influence of the undershot control gate 80 reduces the potentialvariation in the flow pattern through the acoustic flow meter assembly20.

In FIG. 11 the guillotine control gate 80 of FIG. 10 is replaced with atilt-lift type gate 90. The control gate 90 can be separate from theacoustic flow meter assembly 20, as shown, or could be integrated into acombined assembly. Gate 90 allows the gate to be in vertical dispositionwhen closed on seal 82 and an angular or horizontal disposition when inthe open position. Gate 90 is held between a frame 92 which includes ahorizontal track 94 and a vertical track 96. Pins or rollers 98, 100 arelocated on the corners of gate 90 and are held captive in tracks 94, 96.The pins or rollers 98, 100 will move along their respective tracks toallow opening and closing of gate 90. Movement of gate 90 is controlledby a motor driven or hydraulic arm (not shown) coupled to the top 102 ofgate 90. By pulling or pushing the top 102 of gate 90 the gate will beraised or lowered to act as an undershot gate.

The tilt lift gate 90 allows for both a repeatable flow streamline for agiven gate position as well as keeping the velocity of the fluid at thesurface to a minimum. Both the above ensure minimal error in computingthe flow for the segment between the sensors 48 in the top sensor planeelevation 62 and the water surface 84. The undershot gate 90 beinglocated downstream creates a surface velocity distribution through thebody of the acoustic flow meter assembly 20 meter that is morerepeatable and predictable than would be the case if the undershot gate90 were not present. The gate 90 forces the flow to be non-turbulent andlaminar. The gate 90 allows creation of a flow computation algorithmwhich is a function of the gate position and the velocities measured bythe acoustic transducers 46.

The open channel and closed conduit implementations of the acoustic flowmeter assembly 20 are supplied as a single assembly which completelydefines its own geometry such that in-field commissioning of metergeometry parameters is not required.

FIGS. 12 to 17 show a further variation of the tilt-lift gate 90 shownin FIG. 11. In this embodiment gate 120 does not have the pins orrollers 98, 100 at both ends of gate 90 in FIG. 11. The control gate 120can be separate from the acoustic flow meter assembly 20 or could beintegrated into a combined assembly, as shown. The integration of thecontrol gate 120 and the acoustic flow meter assembly 20 will allow adrop in solution which has already been calibrated. Top 122 of gate 120is pivotally mounted by brackets 124 and axle 126. The axle 126 runs inguiding tracks 128. Horizontally mounted arm members 130 are pivotallymounted to axle 126 and will allow gate 120 to be moved from a closed toan open position and vice versa. The arm members 130 can be moved by anelectric motor or hydraulic means depending on requirements. In thisembodiment the arm members 130 are cable driven by spools 132 which arecoupled to an electric motor 134. A gear box 136 will drive the spools132. The cables from spools 132 will be attached to the arm members 130or axle 126.

The positioning of the gate 120 is controlled by an extension arm 138attached to the underside 140 of gate 120. Extension arm 138 has a pivotpoint 142 at its free end. The pivot point 142 is at a position thatwill result in a minimal force (actuation force) to open gate 120. Thiswill result in a low cost actuation and drive train system 132-136. Thepreferred pivot point location is that of the line of the net resultantforce when the gate is in the closed position, typically ⅔ the depth ofwater below the water surface level. This point represents the neutralaxis about which the net forces above the axis equal the net forcesbelow the axis. The force on the gate 120 is due to water pressure andwhich equals:

-   -   ρ*g*h, at a given depth h below the water surface [0092] Where        [0093] ρ is the specific weight of the fluid; [0094] g is the        acceleration due to gravity

The pivot point is offset perpendicular from the underside 140 of gate120. Pivot point 142 is constrained to move along a rail or slot 144which is at a downward angle towards gate 120. The offset assists inproviding a downward force when closing the gate from its fully opensubstantially horizontal position. The offset also ensures the gate sideseals (not shown) do not cross the rail or slot 144 in order to avoidleakage around the side seals. The angle of the rail or slot 144 alsoassists with the downward force when closing gate 120 from its fullyopen substantially horizontal position.

In order to minimise leakage a seal 146 is provided on the free end edgeand sides of the gate 120. The seal is in the form of a bulb seal whichengages on a slightly raised face 148 on the base 72 and sides 74, 76and of the U-shaped channel 70 when gate 120 is in the vertical i.e.closed position. Seal 146 will undergo minimal compression when incontact with the U-shaped channel 70.

FIG. 18 shows the situation of the downstream gate 120 backing up thewater level 84 through the body of the acoustic flow meter under thesame flow and water depth conditions as FIG. 19 which has no downstreamobstruction. It can be seen that the gate 120 acts to maintain a deeperflow through the body of the meter, such that all transducers aresubmerged below the water surface. In FIG. 19 the water surface drops asthe flow velocity increases through the body of the acoustic flow metersuch that several of the transducers 48 are not submerged below thewater surface. This key advantage, discovered through fluid dynamicsimulations causes the water to back-up in front of it in situationswhere there is no downstream tail water. This depth profile isproblematic as many of the sensor paths will be above water and so notable to be used in the measurement. A partially open gate locateddownstream of the meter backs the water up so that it flows through thebody of the meter at approximately constant depth such that moremeasurement paths can be used. This allows the flow meter to be used inhydraulic conditions which would otherwise not be compatible withmetering using this approach.

Variations can be made to the embodiments to suit various environmentalor design requirements. The angular position of sensor pairs 48 is notrestricted to horizontal planes and preferred 45 degrees to thecentreline. The sensor pairs 48 can be at angular orientation. Thesensor 48 is not limited to a send and receive device with a matchingpair. Many sensors could receive signals from the one transmit sensor.

In FIGS. 1 to 6 the invention could be incorporated in-situ into anexisting pipeline. Sound transmission tubes 48 could be tapped andwelded onto an existing pipeline rather than providing a separateacoustic flow meter assembly 20 which is inserted into the pipeline. Theassembly would include the cartridges 44 in a modified frame 24.

In FIGS. 1 to 19 the acoustic transducers 46 have been describedtogether with their operation. The acoustic transducers 46 preferablywork in opposing pairs. The acoustic flow meter assembly 20 measures thetravel time of the acoustic signal in a direction upstream 58B, 60B tothe direction of flow 50, and also in a direction downstream 58A, 60A tothe direction of flow 50 as seen in FIGS. 5 a and 5 b. The flow velocitycreates a difference in the sound wave travel times in the upstream anddownstream direction. This travel time difference is recorded and usedto determine the average velocity of the water along the line of theacoustic path.

The time difference is recorded using transducers and circuitry whichtogether have intrinsic time delays which add to the actual travel timeof the acoustic signal. These transducer 46 and circuitry time delaysmust be subtracted from the recorded acoustic signal travel time so thatthe actual travel time of the acoustic signal may be determined.

The transducer 46 and circuit time delays are typically measured in acalibration of the acoustic flow meter assembly 20, and characterized asa numerical constant which is subtracted from the measured acousticsignal travel time to calculate a best estimate of the actual acousticsignal travel time.

Two constants could be determined by calibrating the acoustic signaltravel time measurements in both the upstream and downstream directions.This is not necessary however, as the acoustic signal travel time in theupstream direction is subtracted from the acoustic signal travel time inthe downstream direction, a single calibrated time delay constant issufficient to calibrate the required system measurement. Under zero flowconditions the upstream signal travel time is precisely equal to thedownstream signal travel time. However, due to different circuit andtransducer time delay characteristics in the circuitry used to measurethe travel times in the upstream and downstream directions, the measuredtravel times will not be identical. The difference in the measuredtravel times will reflect the different time delay characteristics inthe circuitry used to measure the upstream and downstream travel times,and can be determined as a single numerical value at an instant in timeby calibrating the measurement system under still water zero flowconditions.

Unfortunately however, the time delays contributed by the transducers 46and the upstream and downstream measurement circuitry are not constant,but are a function of environmental influences such as temperature andpressure, and of electronic circuit conditions such as operating voltageand temperature. Changes in these time delays result from changes intemperature, pressure, operating voltages and other environmentaldisturbances. These changes result in a change to the calibration of theflow metering system 20 which results in errors in measuring the precisedifference in acoustic signal travel times. This results in errors inthe measurement of flow velocity, which are particularly significant tothe measurement of low flow velocities.

To compensate for changes in the time delays within the upstream anddownstream measurement circuitry, a self-calibrating measurement systemis proposed which is capable of calibrating itself against a referencestandard on every flow velocity measurement, thereby preventing errorsin the measurement of the acoustic signal travel times. Although theembodiment will be described with reference to its operation withirrigation systems the use of this invention is not limited to thatpurpose.

Referring to FIGS. 20 to 25 a measurement system 200 is represented as atimer 202 which has a start input 204 and a stop input 206, togetherwith several signal paths through which electrical information istransmitted. The drawings show only two transducers being represented inthis measurement system 200 namely transducer 46A and transducer 46Bfrom FIGS. 5 a and 5 b for simplicity. All paired transducers 46 fromFIGS. 1 to 20 will be connected in the same manner.

As indicated in FIG. 20, there are electronic system time delays presentin the measurement system 200. These are shown as: [0108] ∂TA is thedelay between a start signal 208 being input to the timer 202 and thecorresponding electrical signal being received by transducer 46A. [0109]∂RB is the delay between the acoustic signal being received bytransducer 46B and the corresponding electrical signal being input tothe stop input 206 of timer 202. [0110] ∂TB is the delay between thestart signal 208 being input to the timer 202 and the correspondingelectrical signal being received by transducer 46B. [0111] ∂RA is thedelay between the acoustic signal being received by transducer 46A andthe corresponding electrical signal being input to the stop input 206 oftimer 202.

The acoustic signal travel time from transducer 46A to transducer 46Balong path 58A is represented as T_(FLOW) _(—) _(A→B) and the acousticsignal travel time from transducer 46B to transducer 46A along path 58Bis represented as T_(FLOW) _(—) _(B→A).

FIG. 21 shows only the signal path when measuring the acoustic signaltravel time from transducer 46A to transducer 46B. This signal traveltime is determined by sending a transmit signal 208 to transducer 46A.This transmit signal 208 has an initial signal characteristic whichdefines the start of the transmit signal. This signal characteristic isinput to the timer 202 and defines the start of the time measurement.The transmit signal 208 is transmitted to transducer 46A which respondsby transmitting an acoustic signal to transducer 46B. Transducer 46Bconverts this acoustic signal to an electrical signal which is inputinto the timer 202 and defines the end of the time measurement. The timemeasured when transmitting the acoustic signal from transducer 46A totransducer 46B isT _(AB)=└(δTA+T _(FLOW) _(—) _(A→B) +δRB)┘

This procedure is then repeated in the opposite signal direction asillustrated in FIG. 22. The acoustic signal travel time from transducer46B to transducer 46A is determined by sending a transmit signal 208 totransducer 46B. This transmit signal 208 has an initial signalcharacteristic which defines the start of the transmit signal. Thissignal characteristic is input to the timer 202 and defines the start ofthe time measurement. The transmit signal 208 is transmitted totransducer 46B which responds by transmitting an acoustic signal totransducer 46A. Transducer 46A converts this acoustic signal to anelectrical signal which is input into the timer 202 and defines the endof the time measurement. The time measured when transmitting an acousticsignal from transducer 46B to transducer 46A isT _(BA)=└(δTB+T _(FLOW) _(—) _(B→A) +δRA)┘

The difference in the sound wave travel times in the upstream anddownstream direction is then measured as

$\begin{matrix}{{\Delta\; T} = {T_{AB} - T_{BA}}} \\{= {\left\lfloor \left( {{\delta\;{TA}} + T_{{{FLOW}\;\_\; A}->B} + {\delta\;{RB}}} \right) \right\rfloor - \left\lfloor \left( {{\delta\;{TB}} + T_{{{FLOW}\;\_\; B}->A} + {\delta\;{RA}}} \right) \right\rfloor}} \\{= {\left( {T_{{{FLOW}\;\_\; A}->B} - T_{{{FLOW}\;\_\; B}->A}} \right) + \left( {\left( {{\delta\;{TA}} + {\delta\;{RB}}} \right) - \left( {{\delta\;{TB}} + {\delta\;{RA}}} \right)} \right)}} \\{= {\left( {T_{{{FLOW}\;\_\; A}->B} - T_{{{FLOW}\;\_\; B}->A}} \right) + X}}\end{matrix}$

Where X is a calibration constant.

In order to calculate the calibration constant the invention providesadditional measurements without using the transducers 46A, 46B. Thisaspect is shown in FIG. 23. The invention switches in an alternativesignal path which bypasses the ultrasonic transducers 46A, 46B to allowthe circuitry time delays to be measured. If the transducers 46A, 46Bare switched out of the circuit and a delay path ∂C is switched in, thenwhen transducer 46A is configured as the transmitting transducer thenthe following equation is applicable:T _(AB) _(—) _(Calibratio n)=[(δTA+δC+δRB)]

This system configuration is shown in FIG. 24.

Similarly, if transducers 46 a, 46B are switched out of the circuit andthe delay path ∂C is switched in then when transducer 46B is configuredas the transmitting transducer then the following equation is alsoapplicable:T _(BA) _(—) _(Calibratio n)=[(δTB+δC+δRA)]

This system configuration is shown in FIG. 25.

These calibration measurements can then be used in conjunction with theacoustic signal travel time measurements to eliminate the circuit delays∂TA, ∂TB, ∂RA, ∂RB from the estimation of the acoustic signal traveltimes such that these travel times can be determined precisely.

The measurement process will be as follows:

1. The measurement system 200 is first configured as per FIG. 21 tomeasure T_(FLOW) _(—) _(A→B).

2. The measurement system 200 is then configured as per FIG. 22 tomeasure T_(FLOW) _(—) _(B→A).

3. The measurement system 200 is then configured as per FIG. 24 tomeasure T_(BA) _(—) _(Calibration)

4. The measurement system 200 is then configured as per FIG. 25 tomeasure T_(AB) _(—) _(Calibration)

The four system measurements are then combined to determine the result(T_(FLOW) _(—) _(A→B)-T_(FLOW) _(—) _(B→A)).

If the calibration times are subtracted from the flow measurement times,then the results areT _(AB) −T _(AB) _(—) _(Calibratio n)=└(δTA+T _(FLOW) _(—) _(A→B)+δRB)−(δTA+δC+δRB)┘=[T _(FLOW) _(—) _(A→B) −δC]T _(BA) −T _(BA) _(—) _(Calibratio n)=└(δTB+T _(FLOW) _(—) _(B→A)+δRA)−(δTB+δC+δRA)┘=[T _(FLOW) _(—) _(B→A) −δC]

The difference in transmit time can then be determined as

$\begin{matrix}{{\left( {T_{AB} - T_{{AB}\;\_\;{Calibratio}\mspace{14mu} n}} \right) - \left( {T_{BA} - T_{{BA}\;\_\;{Calibratio}\mspace{14mu} n}} \right)} = {\left\lbrack {T_{{{FLOW}\;\_\; A}->B} - {\delta\; C}} \right\rbrack -}} \\{\left\lbrack {T_{{{FLOW}\;\_\; B}->A} - {\delta\; C}} \right\rbrack} \\{= \left\lbrack {T_{{{FLOW}\;\_\; A}\;->B} - T_{{{FLOW}\;\_\; B}->A}} \right\rbrack}\end{matrix}$

It can be seen in the above formula that the electronic circuit delaytimes have been removed from the acoustic signal transit timemeasurements, and the difference in signal transit time measurements isdetermined precisely. With high speed computer technology thecalibrations can occur in real time or the calibrations may be monitoredat predetermined intervals.

The invention in another aspect provides a further method of measurementof velocity of fluid flowing in a pipe. In the conventional applicationof acoustic transit time technology to measure the rate of flow in pipesit is common to use either a single path or cross path technique. Theseapplications rely on the pipe being full or pressurized. The single pathtechnique assumes a symmetrical velocity distribution around the centreline of the pipe with oppositely facing and offset top and bottomacoustic transducers. The cross path technique is used where thevelocity distribution is non-symmetrical around the pipe centre line. Inthis cross path technique two pairs of oppositely facing and offset topand bottom acoustic transducers are used and their acoustic paths crossthe pipe centre line. Many flow meter applications not only require theability to detect the real time failure of a flow meter but also theability to record flow measurement without any loss of continuity ofdata. This is especially a requirement of meters that are used forrevenue billing applications with strong quality compliancerequirements. It also applies to meters that are remotely located andcan take some time to service. Accordingly, the failure of an acoustictransducer in the non-symmetrical velocity distribution will result ininaccurate readings as the resulting single path technique will onlyprovide accurate readings in a symmetrical velocity distribution.

In FIGS. 26 and 27 there is shown a pipe 250 with a fluid flowingtherethrough in direction 251. Six pairs of acoustic transducers252,254; 256,258; 260,262; and 264,266 with two pairs of acoustictransducers hidden by their alignment with transducers 260-266 areequi-spaced around pipe 250. The positioning of the acoustic transducersis not restricted to being equi-spaced but can be placed in positions tosuit requirements. The number of pairs of acoustic transducers can varybut at least three pairs must be provided. The upstream and downstreamacoustic paths 270-276 and hidden paths all cross at a central point 278along the central axis 280 of pipe 250. Accordingly, measurements alongthe six paths 270-276 and hidden paths can be made to increase accuracy.If one of the acoustic transducers 252-266 or the hidden transducersfails, then measurements can still be made with the remaining acoustictransducers. The failure can be detected and the faulty acoustictransducer replaced at a convenient time.

This aspect of the invention provides at least three single or crosspaths located around the centre line 280 of pipe 250. This approach willprovide at least three independent flow meters formed by theco-operating pairs of acoustic transducers on pipe 250. The result is toallow the real-time detection of the failure of any one of theindependent flow meters, but also to be able to maintain flowmeasurement until the fault is corrected. To achieve this effect usingother metering technologies, for example, magnetic flow meters, wouldrequire three meters to be installed in series along a section of pipe.

It is evident to the man skilled in the art that the embodiment shown inFIGS. 26 and 27 can be readily incorporated into the embodiment shown inFIGS. 1 to 6.

In accordance with a further preferred aspect of the present invention,and in this regard reference is made to FIGS. 28 to 35 inclusive of thedrawings, what is referred to hereinafter as a time of flight or transittime measuring apparatus is located immediately upstream of a slide orcontrol gate 500. The control gate 500 may be of the type referred to inthe present applicant's Australian Patent No. 2001283691, as referred toand described earlier in this specification.

As shown in FIGS. 28 and 29, preferably the measurement apparatus willtake the form of a conduit 600, of any cross-section but moreparticularly of either a circular, as in FIG. 29, or a parallelepipedalas in FIG. 28, cross-section, which conduit 600 will be associatedwith—either fixedly or removably—the frame of a flow or control gate500.

In FIGS. 28 to 36 there is shown a control gate 500 to be located withina conduit, as for example an irrigation channel (not shown), thefunction of the control gate being to allow a controlled flow of waterthrough the channel. The control gate 500 includes a gate leaf 501 whichslides within a frame 502. Frame 502 has an outer frame member, whichmay be permanently secured to floor and sides of an irrigation channelor conduit and an internal frame member which slides within that outerframe member. The internal frame member may be connected to andseparated from the external frame member with no requirement toundertake civil works on the floor and sides of the irrigation channel.This type of internal/external frame mechanism is further detailed inthe specification of the present applicant's International (PCT) PatentApplication No. PCT/AU2001/001036, the contents of which are includedherein by reference. Gate leaf 501 may be raised or lowered by a liftingmechanism 503 of any known type, as for example that illustrated anddescribed in the present applicant's International Patent ApplicationNo. PCT/AU2010/000115. It should be understood, however, that theinvention is not limited to usage only with such a flow or control gate.

A typical installation would involve a control or flow gate (of anygiven type) with the associated measuring apparatus 600 attached, in anyknown manner and using any known means, to the upstream inlet of aconduit or pipe, located for example in a canal, reservoir or the likewatercourse. In an alternative installation there may be providedconduit connection means at both upstream and downstream ends or sidesof the overall flow meter assembly as referred to earlier in thisspecification.

The conduit 500 has associated therewith acoustic transducers 46 for thegeneration of acoustic beams which traverse the flow through thatconduit 500.

It should be understood that conventional or traditional transit timeflow measurement apparatus have, for their operation, prescribedconditions both upstream and downstream of the measuring device in orderto ensure that there is minimal disturbance to flow. These prescribedconditions are set out in detail in, for example, Australian StandardAS747.

The arrangement in accordance with the present invention relies for itsoperation on a derived relationship between the flow through the conduitand the transit time measurements of acoustic beams which traverse thefluid. The relationship further relies on the measurement inputs ofwater level (as determined by the level sensors) and gate position. Inthat regard reference is also made to the present applicant'sInternational Patent Application No. PCT/AU2002/000230.

In practice the number of acoustic beams which traverse the flow can besingular or many, and can exhibit a variety of different orientations.However, preferred arrangements as shown in the drawings will includethree (3) pairs of acoustic transducers 46 for the parallelepipedalconduit 600 of FIG. 28 and one (1) pair for the circular conduit 600 ofFIG. 29.

The relationship between the flow and transit time, gate opening andwater level may be derived using data flow experiments as explained indetail in the present applicant's International (PCT) Application No.PCT/AU2002/000230 entitled “Fluid Regulation”.

The arrangement is such that the conduit 600 is substantially fixedwithin the channel, whilst the control gate leaf 501 is movablesubstantially vertically within that channel, whereby to allow forvariation of flow through the conduit 600. The arrangement utilises adouble seal 601, see in particular FIGS. 33 to 36, which runs the entirecircumference of the gate 500. That double seal 601 ensures completesealing of the conduit 600 from both upstream and downstream thereof, aswell as external thereto. The gate 500 employs a flat face or surface onboth the upstream and downstream sides of the leaf 501 to ensureposition sealing through the full travel of the gate 500.

With conventional/traditional gate valve designs a bonnet is included inthe overall assembly for purposes of enclosing the gate within aconduit, protecting against leakage. With the arrangement in accordancewith the present invention, utilising a double seal of the type referredto earlier, there is no need for a bonnet or the equivalent.

In the embodiments of FIGS. 37 and 38 there is shown a variation of theembodiment of FIG. 28 with dividers 602. FIG. 37 has a single divider602 whilst FIG. 38 has a pair of dividers 602. The dividers 602 have aplurality of acoustic transducers 46 attached on either side whichcooperate with the acoustic transducers 46 on the inner opposing wallsof conduit 600. As is evident from FIG. 33 the acoustic path lengths ofthe embodiment shown in FIG. 28 will be reduced as the acoustictransducers of the embodiment shown in FIG. 37 will be between thedivider 602 and the inner walls of conduit 600 on either side.Similarly, for the embodiment of FIG. 38 the acoustic path lengths willbe further reduced because the acoustic path lengths are between thedivider 602 and the inner walls of conduit 600 on either side andbetween the dividers 602 in the middle of conduit 600. This reducedacoustic path length will allow a reduction in the length of conduit600. It is possible to have further dividers 602 but the cost of theadditional acoustic transducers 46 would be expensive and notjustifiable.

The embodiments shown in FIGS. 39 and 40 there is shown a variation ofthe embodiment of FIG. 29 with dividers 602. The dividers 602 operate inthe same manner as that described with respect to FIGS. 37 and 38. Againthe resulting reduction in acoustic path length will allow a reductionin the length of conduit 600.

The embodiment shown in FIG. 41 is similar to the embodiment of FIG. 28.The difference between the embodiments is the slanting of the slide orcontrol gate 500. The angling rearwardly of the slide or control gate500 reduces the headroom required when installing the system. FIGS. 42and 43 relate to the use of dividers 602 for the embodiment of FIG. 41and operate identically to the embodiments of FIGS. 37 and 38 previouslydiscussed.

FIGS. 44 to 46 show a schematic drawing of a further measurement systemin the form of an acoustic transit time flow meter designed to measurefluid flows 700 which does not require linked cabling to connect allacoustic transducers 46 to a central location. The measurement system200 described in FIGS. 20 to 25 requires cabling which traversesopposite sides of the open channel. The system shows a left river orchannel bank 702 and an opposite right river or channel bank 704.Conventionally, cabling would be required to cross the river or channelbed 706 between banks 702, 704. It may not be feasible to dig up or cutinto the river or channel bed 706 to lay the required cables. Thisembodiment allows no cabling to be used or limit the cabling to bedisposed along each of the banks 702 and 704 where it can be readilyinstalled. Acoustic transducers 46 are schematically shown attached tothe banks 702, 704 for ease of description but it is understood thatthey could also be contained in cartridges 44A as previously describedand inserted into a flow meter assembly 20 installed in the river orchannel.

In order to be self contained the cartridges 44A may contain theacoustic transducers 46 as previously described. The cartridge 44Acontains the required electronics and processing circuitry and ispowered by a solar panel 708. A telemetry radio 712 allows generation ofRF signals which can be sent and received using data radio antenna 710.Data can also be sent to a central location for storage and furtherprocessing.

FIG. 44 shows use of the transit time flow meter where the transit timeflow meter measures flows by the standard transit time method. The flowmeter consists of two or more cartridges 44A which provide their ownpower supply 708, a shared radio communications link, the acoustictransducers 46, and a synchronising radio signal which is used tosynchronise the signal sampling system clock in each cartridge 44A.

As a minimum, two cartridges 44A are installed—one on either side ofeach bank 702, 704. Four cartridges 44A may also be installed as shownin FIG. 44, two per side to provide the standard crossed-path meteringarrangement. Further cartridge pairs may be used to provide additionalvelocity information within the flow channel.

The cartridge pairs 44A act alternately as an acoustic transmitter andan acoustic receiver. For example, cartridge 714 in the pair acts as atransmitter, and cartridge 716 acts as a receiver and receives theacoustic signal 718 transmitted by cartridge 714. Cartridge 714 recordsthe time of the firing event in its high resolution timing circuitry,and cartridge 716 records the time of the receive event in its highresolution timing circuitry. The timing circuitry in each cartridge is ahigh speed binary counter, which is initialised to a zero value and thenproceeds to count upwards. Each count in these counters is updated in a10 pico-second period, and so a single counter increment represents a 10pico-second duration. The transmit event is captured by circuitry incartridge 714, and the timing count value at this instant is stored in aregister in cartridge 714. The receive event is captured by circuitry incartridge 716 and the timing count value at this instant is stored in aregister in cartridge 716. However, the counter in cartridge 714 is notsynchronised with the counter in cartridge 716, and so the timedifference between the register value stored in cartridge 716 andcartridge 714 is indeterminate. In order to synchronise the timeregister value in each cartridge, an RF synchronisation pulse istransmitted from cartridge 714 to cartridge 716 prior to the firingpulse. This RF pulse travels between the two cartridges 714, 716 at thespeed of light (3×10⁸ m/s), meaning that the time elapsed for acartridge spacing of 100 m is 333 ns. This RF pulse is captured by bothtiming systems in cartridges 714, 716 and provides a common time tagwith which to refer the firing event and receive event within the twocartridge timing circuits. The acoustic transit time is then calculatedby subtracting the firing event time from the receive event time. Thecartridges 714, 716 then swap roles and the transmitter cartridge 714becomes the receiver cartridge and vice-versa. The acoustic transit timein the reverse direction is then calculated, allowing the differentialtransit time to be recorded and used to deduce flow rate through thechannel.

FIG. 45 replaces the RF system of FIG. 44 with a laser system. A syncpulse laser radio 720 (FIG. 47) could then be used as a substitute. Thecartridge 44A shows both options but it is to be understood that thesystem can operate with only one of these options.

The invention will be understood to embrace many further modificationsas will be readily apparent to persons skilled in the art and which willbe deemed to reside within the broad scope and ambit of the invention,there having been set forth herein only the broad nature of theinvention and specific embodiments by way of example.

We claim:
 1. A method of measuring fluid velocity in a pipe or open channel, said method including the steps of: providing a timing circuit which includes a first circuit having at least one upstream acoustic transducer and a second circuit having at least one downstream acoustic transducer; measuring a time delay in detecting an acoustic signal from said at least one upstream acoustic transducer to said at least one downstream acoustic transducer from said first circuit; measuring a time delay in detecting an acoustic signal from said at least one downstream acoustic transducer to said at least one upstream acoustic transducer from said second circuit; measuring a time delay in said first circuit when said at least one upstream acoustic transducer is bypassed in said first circuit; measuring a time delay in said second circuit when said at least one downstream acoustic transducer is bypassed in said second circuit and calculating said fluid velocity using said measurements.
 2. A method of measuring acoustic transit times in an open channel or river, said method including the steps of: providing a first circuit having at least one upstream acoustic transducer on one side of said open channel or river and a second circuit having at least one downstream acoustic transducer on an opposite side of said open channel or river, said first and second circuits including respective timing circuitry which are not synchronised with one another, each of said timing circuitry measuring their respective signal transmit and receive events, at least one of said first or second circuits including an RF or laser to provide synchronising signals between said first and second circuits, wherein an RF or laser synchronising signal is transmitted between said first and second circuits prior to an acoustic signal being transmitted from one of said acoustic transducers between said first and second circuits whereby said RF or laser synchronising signal allows synchronisation between said respective timing circuitry of said first and second circuits of said acoustic signal.
 3. The method of claim 1, wherein a first cartridge contains said first circuit and a second cartridge contains said second circuit.
 4. The method of claim 3, wherein said first and second cartridges are interchangeable thereby allowing said first circuit to be downstream and said second circuit to be upstream.
 5. The method of claim 1, wherein said first and second cartridges are within a frame with a predetermined geometry.
 6. The method of claim 5, wherein said frame includes at least two user accessible ports, one of said user accessible ports containing said first cartridge and another of said user accessible ports containing said second cartridge.
 7. The method of claim 1, further comprising coupling a hollow tube at either end to said pipe or open channel to determine said fluid velocity through said pipe or open channel.
 8. The method of claim 7, wherein each acoustic transducer is located at one end of a respective sound transmission tube and an other end of said respective sound transmission tube opens into said hollow tube.
 9. The method of claim 8, further comprising filling each sound transmission tube with an acoustic transmissive material.
 10. The method of claim 9, wherein a boundary interface is between said acoustic transmissive material in said each sound transmission tube and fluid flowing in said pipe or open channel, said boundary interface formed of a material of suitable acoustic properties to enable ready transmission of said acoustic signals.
 11. The method of claim 8, wherein each sound transmission tube contains fluid from said hollow tube.
 12. The method of claim 2, wherein a first cartridge contains said first circuit and a second cartridge contains said second circuit.
 13. The method of claim 12, wherein said first and second cartridges are interchangeable thereby allowing said first circuit to be downstream and said second circuit to be upstream.
 14. The method of claim 12, wherein said first cartridge contains an RF antenna to provide said synchronising signal.
 15. The method of claim 12, wherein said first and second cartridges are within a frame with a predetermined geometry.
 16. The method of claim 15, wherein said frame includes at least two user accessible ports, one of said user accessible ports containing said first cartridge and another of said user accessible ports containing said second cartridge.
 17. The method of claim 12, further comprising calibrating said first and second cartridges.
 18. The method of claim 17, wherein said first and second cartridges are calibrated with a calibration referenced to a position relative to said open channel or river.
 19. The method of claim 2, further comprising lowering a gate downstream of said first and second circuits to influence surface velocity of at least some fluid in said open channel or river.
 20. The method of claim 19, wherein said gate forces said fluid in said open channel or river to be non-turbulent. 